[fuchsia] Stop overriding toolchain shadow-call-stack setting.

This is a partial reland of 4189e014
The original change removed both shadow-call-stack and safestack
overrides, which broke Blink heap unit-tests under x64, but was not
caught by CQ due to issue 1044935.

Fuchsia builds have been orcibly enabling shadow-call-stack in
advance of it becoming the toolchain default, to gauge size impact.

Enabling shadow-call-stack under x64 is not recommended and breaks code
which uses C++ exceptions, which is true of some V8 code & tests.

Since the toolchain default for shadow-call-stack should now correctly
enable it under ARM64 but not x64, remove the override.

Bug: 1028617, 821951
Change-Id: I8784d8fc24d81716e27a53d41f0548343621ad75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2014908
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734931}

build/config/fuchsia/BUILD.gn

@@ -51,11 +51,7 @@ config("compiler") {
 
   # TODO(crbug.com/821951): Clang enables SafeStack by default when targeting
   # Fuchsia, but it breaks some tests, notably in V8.
-  # Force the toolchain to use shadow-call-stack instead, until that is default.
-  cflags += [
-    "-fno-sanitize=safe-stack",
-    "-fsanitize=shadow-call-stack",
-  ]
+  cflags += [ "-fno-sanitize=safe-stack" ]
 
   libs = [ "zircon" ]
 }